In recent years, there have been produced prototypes of nitride semiconductor laser elements that employ a nitride semiconductor material as exemplified by GaN, InN, AlN, and mixed crystal semiconductors of such materials and that emit light in a blue to ultraviolet region, and there has been paid much attention to, as next-generation high-density information recording/reproducing devices, optical information recording/reproducing devices that employ such a nitride semiconductor laser element as a light source. These optical information recording/reproducing devices adopt, as a method for recording information, for example, a method based on crystal phase change or a method based on magnetic phase transition.
These methods both use the energy of laser light as the heat source for a write operation, and thus, when used as such a light source, a nitride semiconductor laser element is required to operate at a high output. Moreover, for higher recording density, laser light needs to be focused into an extremely small spot, and the so focused small spot of laser light is required to remain in position during a write operation. Accordingly, a nitride semiconductor laser element used as a laser light source is required to oscillate in the fundamental lateral mode up to a high output.
For example, Collection of Lecture Manuscripts Prepared for the 47th Confederated Lecture Meeting for Applied Physics 31p-YQ-8 and an article by Shin-ichi Nagahama et al. contained in Japanese Journal of Applied Physics (Jpn. J. Appl. Phys.) Vol. 39 (2,000), pp. L647-L650, include reports on nitride semiconductor laser elements that oscillate in the fundamental lateral mode continuously up to an output of 30 to 40 [mW] at room temperature.
An example of such a conventional nitride semiconductor laser element is shown in FIG. 14. FIG. 14 is a sectional view schematically showing the structure of a principal portion of a nitride semiconductor laser element that oscillates at a wavelength of 410 [nm]. The nitride semiconductor laser element shown in FIG. 14 has the following layers formed one on top of another in the order mentioned on the surface of an n-type GaN substrate 171 (with a film thickness of 30 to 300 [μm]): an n-type In0.05Ga0.95N buffer layer 172, an n-type Al0.05Ga0.95N clad layer 173 (with a film thickness of 0.5 [μm]), an n-type GaN optical waveguide layer 174 (with a film thickness of 0.1 [μm]), an active layer 175 (with a film thickness of 360 [Å]), a p-type Al0.2Ga0.8N layer 176 (with a film thickness of 200 [Å]), a p-type GaN optical waveguide layer 177 (with a film thickness of 0.1 [μm]), a p-type Al0.05Ga0.95N clad layer 178 (with a film thickness of 0.5 [μm]), and a p-type GaN contact layer 179 (with a film thickness of 0.2 [μm]).
Here, the active layer 175 is composed of In0.2Ga0.8N well layers each with a film thickness of 40 [Å] and n-type In0.05Ga0.95N barrier layers each with a film thickness of 80 [Å], formed alternately one on top of another in three periods. Part of the p-type GaN contact layer 179 is masked, and then the p-type AlGaN clad layer 178 and the p-type GaN contact layer 179 are etched to form a ridge-shaped stripe with a width of 2 μm. This nitride semiconductor laser element has a waveguide structure in which the active layer 175 and the optical waveguide layers 174 and 177 are sandwiched between the clad layers 173 and 178. Thus, the light generated in the active layer 174 is confined in this waveguide structure to cause laser oscillation.
Substantially over the entire surface of the so etched region, there is formed an insulating film 180. Then, a positive electrode 181 is formed substantially over the entire surface of the insulating film 180, which includes the entire surface of the portion of the p-type GaN contact layer 179 that is exposed in the form of a surface. Moreover, a negative electrode 182 is formed substantially over the entire bottom surface of the n-type GaN substrate 171. Both end surfaces of the stripe function as mirrors, and thus form an optical cavity.
In a case where a nitride semiconductor laser element of this type is used as a light source that is indispensable to record and reproduce information to and from an optical disk, it is typically fed with a pulse current as its drive current. Specifically, for reproduction, an optical output of several [mW] suffices, but the return light noise needs to be minimized. One way to achieve this is to intentionally lower the use efficiency of laser light. This may reduce the amount of return light to a certain degree and thus help reduce the resulting noise below a permissible level. A more common way to reduce the return light noise, however, is to reduce the coherence of the nitride semiconductor laser element to make less likely the coupling of the return light with the light inside the laser. For this purpose, a nitride semiconductor laser element is commonly driven with a pulse current having a high frequency of 100 [MHz] superimposed thereon.
For recording also, typically, a pulse current train of which the pulse width has a peak level higher than the output during reproduction is modulated according to a signal. The purpose of using not a DC current but a pulse current here is as follows. If a DC current is modulated according to a signal, when the signal is long, over the duration thereof, i.e., from the beginning toward the end thereof, the temperature on an optical disk rises, with the result that the trace of recording on the disk is thin at the beginning but become increasingly thick toward the end. When information is reproduced from an optical disk having it recorded thereon in such a way, read errors are more likely.
As discussed above, a nitride semiconductor laser element used in an optical information recording/reproducing device is required to pulse-oscillate in the fundamental lateral mode up to an output as high as 20 to 35 [mW] on a time average basis. Accordingly, for example, in a case where it is driven with a pulse current with a duty factor of 30% (what a duty factor means will be described later), it is required to pulse-oscillate in the fundamental lateral mode up to a peak output of 60 to 100 [mW] within the pulse width duration.
However, when the inventors of the present invention produced 100 conventionally structured nitride semiconductor laser elements and drove them with a pulse current with a duty factor of 30%, it was confirmed that only less than 10 of them oscillated in the fundamental lateral mode at peak outputs of 0 [mW] to 60 [mW] within the pulse width duration, and that the rest oscillated in the high-order lateral modes in the above-mentioned range of outputs. This indicates that the yield rate of conventionally structured nitride semiconductor laser elements is 10% or less.
FIG. 12 shows an example of the peak output plotted against the peak current within the pulse width duration (hereinafter referred to simply as the “I-L characteristic”) as observed in a conventional semiconductor laser element. The thin line in FIG. 12 indicates an example of a linear I-L characteristic of a nitride semiconductor laser element that operates in an ideal fashion; specifically, driving it with a pulse current with a duty factor of 30% results in an I-L characteristic that is linear up to a peak output of 90 mW within the constant current pulse width duration. As the peak level of the pulse current is increased, when the level becomes higher than a threshold level Ith, oscillation in the fundamental lateral mode starts, and as the peak level of the pulse current is further increased, the peak output linearly increases up to 90 [mW].
By contrast, the thick line indicates an example of the I-L characteristic of the aforementioned more than 90 nitride semiconductor laser elements produced by the inventors of the present invention. This I-L characteristic includes a kink, i.e., a non-linear portion. Specifically, as the peak level of the pulse current is increased, when the level becomes higher than the threshold level Ith, oscillation in the fundamental lateral mode starts, and as the peak level of the pulse current is further increased, when the level becomes higher than a kink current level Ik, a kink occurs.
In all of the aforementioned 90 nitride semiconductor laser elements in which a kink was observed, it was recognized that the horizontal far-field pattern (hereinafter referred to as the “FFP”) changes before and after the point at which the kink was observed in the I-L characteristics. This proves that the cause of the kink is a change in the horizontal lateral mode. Now, with reference to FIG. 13, how the FFP changes against the peak level I of the pulse current will be described. It should be noted that, in FIG. 13, the horizontal FFP is shown as the light intensity distribution at different horizontal angles with respect to the direction in which laser light is emitted from a nitride semiconductor laser element.
FIG. 13(a) shows the horizontal FFP as obtained when a pulse current of which the peak level I fulfils Ith<I<Ik is injected to a nitride semiconductor laser element. Here, a light intensity distribution with a single peak is obtained. FIG. 13(b) shows an example of the horizontal FFP as obtained when a pulse current of which the peak level I fulfils I>Ik is injected. In this example, where the injected current has just exceeded the current level Ik, the horizontal FFP changes to a light intensity distribution with two peaks.
FIG. 13(c) shows another example of the horizontal FFP as obtained when a pulse current of which the peak level I fulfils I>Ik is injected. In this example, where the injected current has just exceeded the current level Ik, the position of the peak of the light intensity distribution of the horizontal far-field pattern is displaced. Here, the degree of the displacement is about 1 to 5 degrees relative to the position of the peak in FIG. 13(a).
In an AlGaAs-based semiconductor laser element, an asymmetric horizontal carrier distribution and an asymmetric horizontal light intensity distribution inside the stripe lead to unstable operation. Accordingly, a conventional way to ensure oscillation at a high output in the single lateral mode is to adopt an optical waveguide of the real refractive index type. An effective way to maintain the fundamental lateral mode up to a high output is to make the width of the optical waveguide narrow. However, making it too narrow results in weakening the horizontal confinement of the active layer (this can be explained by calculating the horizontal electric field distribution). Thus, there exists an optimum value for the stripe width.
In a nitride semiconductor laser element, the optimum stripe width is approximately proportional to the laser oscillation wavelength, and thus, in a nitride semiconductor laser element, the optimum stripe width is comparatively small, specifically about 2.0 [μm]. Moreover, the influence of an error from the optimum stripe width on the characteristics of the nitride semiconductor laser element becomes increasingly striking as the design value of the optimum stripe width is made smaller. In a nitride semiconductor laser element, to sufficiently reduce the influence of such an error on its characteristics, it is necessary to form the stripe so that its actual width is within ±0.1 [μm] of the optimum stripe width, and therefore it is extremely difficult to form it.
Furthermore, in a nitride semiconductor laser element, it is difficult to make flat the end surfaces through which laser light exits therefrom. The most commonly used material of the substrate used in a nitride semiconductor laser element is sapphire. When a multiple-layered structure formed of nitride semiconductor materials is epitaxially grown on top of this sapphire substrate, it grows in such a way that the cleavage plane of the sapphire substrate forms an angle of precisely 30 degrees to the cleavage plane of the nitride semiconductors. That is, the cleavage surface of the sapphire substrate is not parallel to the cleavage surface of the multiple-layered structured formed of nitride semiconductor materials. This makes it extremely difficult to obtain flat cleavage end surfaces. This makes the refractive index at the end surfaces horizontally uneven, and makes the light intensity distribution within the stripe more likely to be horizontally asymmetric, making instability more likely.
Moreover, in a nitride semiconductor laser element, InGaN, which is commonly used as the material of the active layer, is liable to phase separation. If even a slight defect is present in the base material of the active layer, the defect acts as a seed to cause In to segregate to form a layer. This makes it difficult to form the active layer with uniform composition. This makes the carrier distribution and light intensity distribution within the stripe more likely to be horizontally asymmetric, making instability more likely.
Moreover, in a nitride semiconductor laser element, the effective mass of the carriers in the active laser is greater than in a laser element formed of another material such as an AlGaAs-based one. This makes the diffusion length of the carriers accordingly small, and thus tends to aggravate the horizontal asymmetry of the carrier distribution. This, as in the case described above, makes the carrier distribution within the stripe more likely to be horizontally asymmetric, making instability more likely.
For the reasons explained above, a nitride semiconductor laser element is more likely to cause a kink as indicated by the thick line in FIG. 12 at a low optical output than is a semiconductor laser employing another material such as an AlGaAs-based one. Accordingly, simply setting the stripe width at the optimum value is not sufficient for a nitride semiconductor laser element to maintain oscillation in the fundamental lateral mode up to a high output.